A thermostat is a temperature-sensitive switch, or collection of switches, that provide signals to control heating, ventilating, and air conditioning (HVAC) systems based on parameters measured by the thermostat. American thermostats typically provide a simple ON/OFF signal to indicate when cooling or heating is required. For example, a cooling signal will be ON and a heating signal will be OFF when the parameters at the thermostat indicate that the HVAC system should be cooling an area. Likewise, a heating signal will be ON and a cooling signal will be OFF when the parameters at the thermostat indicate that the HVAC system should be heating the area.
In an effort to boost efficiency and gain a competitive advantage, however, many HVAC manufacturers have created HVAC systems that allow for multiple operation stages. These multiple stages allow for different levels of heating or cooling. For example, a two-stage heating system may involve the use of a furnace and a heat pump. At a low heating stage, only the heat pump operates; at a high heating stage, both the heat pump and the furnace operate. Similarly, a two-stage cooling system may involve the use of two compressors. At a low cooling stage, only one of the compressors operates; at a high cooling stage, both compressors operate. In this way, as demand increases, the multiple stage unit can respond by bringing extra capacity online to meet the demand.
Multiple stage systems can use their multiple levels of heating or cooling to save energy. Much like a dimmer on a light switch, the multiple stages allow the system to adjust the level of heating or cooling based on the current circumstances (e.g., outside temperature). As a result, they can use the lowest level of heating or cooling needed and only use higher levels when circumstances require them.
Various HVAC equipment manufacturers have therefore developed multiple-stage systems to meet market demands. However, many manufacturers do not build their own thermostats for use with their equipment. Instead, these manufacturers rely on other thermostat manufacturers to deliver multiple-stage thermostats for use with their equipment. As a result, HVAC equipment manufacturers often have no control over what signals they will receive from the thermostats that they use.
As with many single-stage American thermostats, many multiple stage American thermostats simply provide a simple ON/OFF signal to indicate when different cooling or heating stages are required. For example, two-stage American thermometer may provide four signals: a first-stage cooling signal, a second-stage cooling signal, a first-stage heating signal, and a second-stage heating signal. The first-stage cooling signal will be ON and the other three signals will be OFF when the parameters at the thermostat indicates that the HVAC system should be cooling an area using first-stage cooling; the second-stage cooling signal will be ON and at least the two heating signals will be OFF when the parameters at the thermostat indicates that the HVAC system should be cooling an area using second-stage cooling; the first-stage heating signal will be ON and the other three signals will be OFF when the parameters at the thermostat indicates that the HVAC system should be cooling an area using first-stage heating; and the second-stage heating signal will be ON and at least the two cooling signals will be OFF when the parameters at the thermostat indicates that the HVAC system should be heating an area using second-stage heating.
The information provided by these signals is therefore relatively simple. As a result, conventional multiple-stage HVAC systems often rapidly switch between stages when operating on the border between two stages (e.g., near a temperature threshold). This can cause HVAC equipment to be repeatedly turned on and off in a short time period, lowering the overall efficiency of the entire system.
It would therefore be desirable to provide a system and a method for controlling a multiple-stage HVAC unit that allows for more granularity in the operation of the HVAC unit.